Track Drainage Flashcards
Name different sources of track water
- Precipitation from rain and snow
- Flooding from rivers and canals
- Water from adjacent slopes
- Upward seepage from the subsurface
Why is excess water an issue?
It can lead to excess pore water pressures developing
- can lead to the track experiencing significant deterioration very rapidly
However (as graph shows) moisture is required to achieve a high degree of compaction
Name the different problems that may occur if excess pwp is developed
- General loss of subgrade strength
- Plastic strain accumulation (due to excess pwp under cyclic loading decreasing the track stiffness and formation strength)
- Subgrade attrition and slurry formation from the ballast
- Wetbeds (hydraulic pumping of fine material)
- Volume change of the subgrade from swelling
NB. excess track water is particularly a problem if the material is in a saturated state
Describe the principles of flow
Flow through a porous media may be laminar (steady) or turbulant (unstead)
The flow may be saturated or unsaturated, and we assume that the water is incompressible
What is the equation for the average flow velocity (laminar flow)?
NB. k (coefficient of permeability) also referred to as hydraulic conductivity
What is Darcy’s law, which is used to calculated the quantity of flow q?
NB. unit for flow is (volume/time)
What happens to the ballast (gravel) if it is ‘fouled’?
Also, what are typical k (coefficient of permeability/hydraulic conductivity) values for the different soil types?
If fouled, its permeability can drop significantly
In what situations is water in embankments:
a) not a problem?
b) a problem?
a) in general, ground water is not a problem because the phreatic surface is well below the track
b) it can become a problem in heavy rainfall
- pore pressure can develop in the embankment under train loading
- depends on the permeability of the embankment
What type of material is generally used in embankments to prevent excess pwp development?
Granular material:
- can be well compacted
- acts as a drainage system (preventing excess pwp development)
In what situations is water in level ground:
a) not a problem?
b) a problem?
a) in general, excess pwp development is not a problem if the water table is below the track structure
b_1) if capillary action produces a near saturated soil where strength loss can occur
b_2) ballast and/or subballast becomes contaminated and retains water
b_3) if sufficient track drains are not installed and water can accumulate within the trackbed because of rainfall (ie. it pools in subgrade pockets)
How can track drainage issues occur in cuttings?
- A seepage condition exists because the natural ground water is often higher than the track level (water table intercepts the slope sides)
- Water flows out of the slope sides onto the track
- This causes erosion of the track and slope instability
- Wetbed formation can also cause track geometry issues, as water will also flow upwards underneath the track
What are different drain types trying to achieve?
High permeability
What are two example track drains?
- Pipe drain
- Concrete ditch
NB. both of these types require a drainage path to the drain (see graph)
What are two examples of channel drain types?
French drain type
Pipe drain type
- has a perforated pipe (with geotextile)
Geocomposite channel drains
What are the two types of geocomposite design channel drains, and when can they be used?
Slot type:
- can be cut into the trackside, because there is no backfill
Trench type:
- can be cut into the trackside, but requires granular backfill
What does the installation of lineside drains in cuttings do?
- It lowers the trackbed water table (in cuttings)
- The trackbed will heave due to excavation of the soil
- The resultant swelling increases the void ratio; the soil’s moisture content will increase as water flows into the new voids
To improve track drainage, what else can be considered for application to track in cuttings?
Installing a cross-drained trackbed
